"My theory on who sent the call: 'The Chicken Man' (Gus) has the intense
surveillance guy trailing the cousins (who better to trail them than
that guy? He seems to be very much on top of his clandestine methods).
As we saw in his interactions with Walt, the guy still has a heart
underneath all that military-esque armor. My guess, he trailed the
cousins for sometime and sent out the warning to give the guy a
last-second chance at survival."